Understanding how N-cycling processes in unmanaged grassland vary spatially and seasonally would aid the development of management strategies which capitalize on the behavior of its N cycle, in order to manage N losses when such system is altered. Our objectives were: (1) to quantify gross rates of internal N-cycling processes (i.e. mineralization, nitrification, and immobilization) from an unmanaged grassland, and (2) to investigate the role of topography and climatic factors on the spatial and seasonal variation of these processes. We delineated our study site into three topographic units based on soil and drainage types: upper, lower, and drained lower slopes. The dynamics of NH4(+) in the internal N cycle was influenced by the spatial and seasonal variation of soil microbial biomass, in which the spatial variation resulted from long-term topographic influence and the seasonal variation from seasonal flushes of available organic matter. The drained lower slope had the highest microbial biomass (647 mg C kg(-1) and 90 mg N kg(-1)), gross N mineralization (8 mg N kg(-1) d(-1)), NH4(+) immobilization (6 mg N kg(-1) d(-1)), and fastest NH4(+) turnover (0.5 d), indicating that the drainage favored microbial growth and activity. The seasonal pattern of NH4(+) transformations and microbial biomass showed that the increases in microbial N (in fall and late spring towards summer) were paralleled by high NH4(+) immobilization and a decrease in the microbial C-N ratio. Spatial variation of NO3(-) transformations showed the effect of topography through water redistribution; higher gross nitrification was observed in the upper (1.7 mg N kg(-1) d(-1)) than lower slopes (1.1 mg N kg(-1) d(-1)), with drainage also favoring gross nitrification (1.4 mg N kg(-1) d(-1)) through improved soil aeration. The seasonal pattern of gross nitrification was related to soil moisture (r = -0.79, P less than or equal to 0.01) and temperature (r = 0.55, P less than or equal to 0.05). Gross nitrification accounted 32% of the NH4(+) produced. NO3(-) immobilization was about 50% of NH4(+) immobilization. NO3(-) immobilization was highest when microbial immobilization was most favorable and available NH4(+) was insufficient to meet microbial demand. NO3(-) was rapidly produced and consumed (0.7 d average turnover time), and hence its usually small pool size in unmanaged grassland cannot be used as basis to dismiss its importance in the internal N cycle. Our study supported the concept of considering both the spatial and seasonal variation of soil biochemical processes in refining nutrient management strategies in an ecosystem.